This invention relates to the drawing of wire. Wire is made by drawing down rod and hereinafter rod, partly drawn rod and finished wire are all referred to, for convenience, as a metal filament.
Conventionally, steel wire is drawn from a steel rod by passing it in succession through a number of dies or "holes". The number of dies through which the metal filament is passed will depend on the difference in sizes between the starting rod and the finished wire and also on the draft in each die. A conventional continuous wire-drawing machine may have any number between four, and twelve dies and between each pair of successive dies there is provided a rotating capstan or block which pulls the metal filament through the preceding die and accumulates the metal filament thereon. It will be appreciated that as the metal filament decreased in cross-sectional area so it will increase in length and the blocks will be driven at increasing speeds with the decrease in cross-section of the metal filament.
It is important in the manufacture of wire from many metals, and particularly in the manufacture of steel wire, to prevent the temperature of the metal filament from increasing to such a value that the properties of the wire are adversely affected by strainage hardening which is a time/temperature effect and which decreases the ductility of the filament, particularly the torsional ductility although the longitudinal ductility also decreases, while at the same time increasing the ultimate tensile strength.
In steel wire drawing, the metal filament at each stage of drawing passes through a soap box where a drawing soap is applied to the surface of the filament. The filament then passes through the die and in its passage therethrough is lubricated by the soap which it has previously picked up. From the die, the filament passes to the block which succeeds the die and a number of turns of the filament are built up on the block before the filament passes to the next soap box, die and block.
Conventionally, the blocks of a wire-drawing machine are internally cooled by circulating water through them, the water serving indirectly to cool the filament. Moreover, the more turns of the filament on a block, the longer time has the filament to cool and the better can the temperature be controlled. However, it is necessary to treat the water to prevent it scaling the surface of the passages in the block and the water has to be kept clean otherwise it becomes contaminated. Both scale and scum can reduce the heat transfer through the walls of the block and thus decrease the effeciency of the cooling. Moreover it is necessary to provide a comparatively expensive water treatment and cooling plant to ensure that the water is adequately cooled and that it is free of scum and that it will not scale the inner surface of the block.
It has been found, in practice, that this indirect cooling merely by passing water through internal passages in the blocks is insufficient to keep the temperature of a steel filament below that at which it is damaged by strainage hardening, especially with the increasing speeds of wire drawing which are not being used and with the increasing use of controlled cooled rods rather than lead-patented rods. Obviously it is preferable from a production point of view to be able to increase the speed of drawing but, on the other hand, the higher the speed of drawing the more heat is generated in a given time and the more the filament has to be cooled. Moreover, a lead-patented material can withstand higher temperatures during drawing without deleteriously affecting the properties of the finished wire than can material which has been drawn from controlled-cooled rod. This is because the pearlitic spacing of the lead-patented rod is finer than that of the controlled-cooled rod. Increasing use is being made of controlled-cooled rods because they save an expensive heat treatment operation and therefore it is becoming even more important to control the temperature of the filament being drawn both because of the increasing use of the controlled-cooled rod and because of the increasing speed of wire drawing.
A solution is therefore required to the problem of maintaining the filament at a low temperature during drawing so as to control the strainage hardening which occurs, particularly with high carbon steel wire. We have above been referring specifically to the drawing of steel wire but the problem arises in connection with the drawing of any metal filament where the finished wire is deleteriously affected by heat. It is pointed out, however, that there are some materials, for example tungsten, where it is advantageous to draw the metal hot and of course the present invention is not concerned with the drawing of such materials unless the drawing temperature reaches such a value as deleteriously to affect the mechanical properties of the finished wire.
It is an object of the present invention to provide an apparatus for wire drawing and a method of wire drawing which is simple, comparatively inexpensive and which enables the temperature of the filament being drawn to be readily controlled.